Many devices and most mobile computers today are equipped with wireless infrared facilities for communication links. Traditionally, infrared links have been classified according to whether they employ a directional or non-directional receiver and transmitter, and whether or not they rely upon the existence of an uninterrupted line-of-sight path between the receiver and the transmitter. At present, directed, line-of-sight links, hereinafter abbreviated to LOS, are the most widely used. Because they employ directional receivers and transmitters, the path loss is minimized, and multipath distortion is usually negligible. Another link design is the non-directed, non-LOS link, also referred to as a diffuse link, which relies upon diffuse reflection of light from an extended surface, such as a ceiling and walls.
A unit which is able to transmit and receive infrared signals is called a transceiver. Practical wireless infrared transceivers are restricted to use one optical receiver, which might be a photodiode (PD) and one optical emitter, which might be a light emitting diode (LED). The current types of transceivers based on LOS propagation are best suited for point-to-point communication and are not suited for integration in a mobile- or fixed platform meant to operate in a wireless infrared networking environment. These transceivers commonly contain only one optical receiving element that has a fundamentally different reception characteristic compared to the optical transmitter's characteristic. Such transceivers violate the optical parity rule. Because, the receiver exhibits a reception angle φR of about ±60°, also referred to as wide-angle, and the transmitter comprises an emission angle φE of about ±15°, also referred to as narrow-angle. This leads to both insufficient connectivity coverage and link performance degradation in a typical networking application. Insufficient connectivity coverage means that i) network participants cannot connect to certain other participants, ii) certain links are unreliable, or iii) some links offer not enough bandwidth for the application, which means that the required data rate can not be achieved. Further, the data throughput is low because of the low data rate and/or high error rate which means that performance degradation occurs because of reduced link quality combined with improper operation of a collision avoidance mechanism.
The optical parity concept was disclosed in the contribution “Request for Comments on Advanced Infrared (AIr) IrPHY Physical Layer Specifications”, Standards contribution to Infrared Data Association (IrDA), Toronto, Canada, Apr. 15-17, 1997, Version 0.1 (Hewlett-Packard Company and IBM Corporation).
An U.S. patent application with the Ser. No. 048 749 US, filed on 26 Mar. 1998 and entitled “Optoelectronic Transceiver”, discloses a concept of optical transceiver parity. This US patent application is presently assigned to the assignee of the instant application.
U.S. Pat. No. 5,566,022 is related to an infrared communication system. The system includes a plurality of infrared transceivers for receiving and transmitting infrared signals through the free air. A circuit determines the direction of arrival of the received signal and provides this information to a dedicated logic controller (DLC), for registration purposes and for controlling the respective infrared transmitter.
One of the important features of infrared communications is its sensitivity to the direction of reception. The publication “Direction Diversity for Indoor Infrared Wireless Communication Receivers” by M. R. Pakravan and M. Kavehrad of the IEEE International Conference on Communication, Jun. 18-22, 1995, Seattle, discusses the effects of rotation on the characteristics of the received signal from a simulation point of view.
The article “Design Considerations for Broadband Indoor Infrared Wireless Communication Systems” by M. R. Pakravan and M. Kavehard in International Journal of Wireless Information Networks, Vol. 2, No. 4, 1995, is similar to the publication mentioned above and discusses the effects of receiver direction and field-of-view on the channel parameters.
In the paper “Wireless Infrared Communication Links using Multi-Beam Transmitters and Imaging Receivers” by A. P. Tang, J. M. Kahn, Keang-Po Ho, of the IEEE International Conference on Communication, Jun. 23-27, 1996, Dallas, the use of imaging receivers in infrared links is analyzed.
The research report “Angle Diversity for Nondirected Wireless Infrared Communication” by J. B. Carruthers and J. M. Kahn, University of California, Berkeley, submitted to IEEE Transactions on Communications, discusses practical considerations for multi-element angle-diversity systems. Unfortunately, the report does not offer a practical solution to the present problem because it is based on highly complex and costly optical receiver arrays combined with analog high-order signal selection/concentration schemes.
The article “Angle Diversity to Combat the Ambient Noise in Indoor optical Wireless Communication Systems” by R. T. Valadas, A. R. Tavares, A. M. de Oliveira Duarte, in International Journal of Wireless Information Networks, Vol. 4, No. 4, 1997, describes theoretical approaches to estimate several signal-to-noise ratios based on the analog current of several photodiodes.
In the article “Signal Processing of High Speed Nondirective Infrared Wireless Communications” by Po-An Sung, Ya-Ku Sun, Kwang-Cheng Chen, in Journal of the Chinese Institute of Electrical Engineering, Vol. 2, No. 4, 1995, theoretical and numerical results of different diversity techniques are demonstrated.
All mentioned documents describe several theoretical approaches and simulations but these give no practical solution for known technical problems.
Further, network connectivity coverage between mobile platforms, e.g. laptop computers, and fixed access points, e.g. repeater stations or printers, that are all equipped with conventional wireless infrared transceivers is expected to be insufficient for typical user scenarios. Usually, one single transceiver is used in mobile or certain fixed platforms which leads to the above mentioned problems and disadvantages in wireless optical networking application. Some platforms, e.g. laptop computers, are equipped with two transceivers and the user has to decide by manual interventions which one of the two transceivers is to be used. Thus, current infrared transceivers are limited for the use of future wireless infrared applications based on multi-point connectivity.